Two years of support for this project will allow us to perform the experiments proposed in Aims 3 and 4 of the original application. We have been able to complete the bulk of Aims 1 and 2 with support from the American Heart Assoc. This supported our hypothesis that simultaneous exposure of supraoptic neurons (SON) to ATP and phenylephrine induces a shift to non-desensitizing ionotropic purinergic receptor subtypes that allow ATP to sustain vasopressin (VP) release for hours. This work has been submitted for publication. The AHA grant does not provide support for Aims 3 and 4 which are crucial to understanding the mechanism by which norepinephrine can induce the shift in purinergic receptor subtype: Aim 3 is focused on identifying the a1-adrenergic receptor subtype activated by phenylephrine in SON and Aim 4 will identify the signal cascades activated by phenylephrine in SON. Identifying the a1-R signal cascade is critical to understanding why ATP activation of metabotropic purinergic receptors does not induce the same effect as phenylephrine. The ability of phenylephrine to induce sustained responses to ATP is important, because it provides a mechanism for homeostatic CNS regulatory systems to mount extended responses in order, for example, to restore blood volume following hemorrhage or for sustained arousal.